Ion-beam excitation of liquid argon

The scintillation light of liquid argon has been recorded wavelength and time resolved with very good statistics in a wavelength interval ranging from 118 nm through 970 nm. Three different ion beams, protons, sulfur ions and gold ions, were used to excite liquid argon. Only minor differences were observed in the wavelength-spectra obtained with the different incident particles. Light emission in the wavelength range of the third excimer continuum was found to be strongly suppressed in the liquid phase. In time-resolved measurements, the time structure of the scintillation light can be directly attributed to wavelength in our studies, as no wavelength shifter has been used. These measurements confirm that the singlet-to-triplet intensity ratio in the second excimer continuum range is a useful parameter for particle discrimination, which can also be employed in wavelength-integrated measurements as long as the sensitivity of the detector system does not rise steeply for wavelengths longer than 190 nm. Using our values for the singlet-to-triplet ratio down to low energies deposited a discrimination threshold between incident protons and sulfur ions as low as ∼2.5 keV seems possible, which represents the principle limit for the discrimination of these two species in liquid argon

Light emission from heteronuclear Ar-Kr doubly ionized excimer molecules

Light emission from a  ~100 mbar Ar-Kr mixture excited by a pulsed discharge is described. The discharge was arranged to form a homogeneous cathode layer and spatial filtering was used to measure time-dependent spectra emitted from a region where electrons accelerated in the cathode sheath induce the light emission. Novel excimer bands were observed around a wavelength of 315 nm in addition to the better known so called third excimer continua of Ar and Kr. A tentative assignment for these bands to charge-transfer transitions: ArKr2+ →  Kr+ +  Ar+ + hν is provided and discussed in the context of earlier works on heteronuclear ionic excimer molecules. Predictions for the wavelength positions of similar emission bands are provided for other combinations of noble gases. The rate constants for the formation of heteronuclear ArKr2+ excimers in three-body reactions and two-body collisional quenching of Kr2+∗ ions by Ar atoms have been determined from the time dependence of the ArKr2+ emission